Micro-/Nanorobots Propelled by Oscillating Magnetic Fields

Abstract

Recent strides in micro- and nanomanufacturing technologies have sparked the development of micro-/nanorobots with enhanced power and functionality. Due to the advantages of on-demand motion control, long lifetime, and great biocompatibility, magnetic propelled micro-/nanorobots have exhibited considerable promise in the fields of drug delivery, biosensing, bioimaging, and environmental remediation. The magnetic fields which provide energy for propulsion can be categorized into rotating and oscillating magnetic fields. In this review, recent developments in oscillating magnetic propelled micro-/nanorobot fabrication techniques (such as electrodeposition, self-assembly, electron beam evaporation, and three-dimensional (3D) direct laser writing) are summarized. The motion mechanism of oscillating magnetic propelled micro-/nanorobots are also discussed, including wagging propulsion, surface walker propulsion, and scallop propulsion. With continuous innovation, micro-/nanorobots can become a promising candidate for future applications in the biomedical field. As a step toward designing and building such micro-/nanorobots, several types of common fabrication techniques are briefly introduced. Then, we focus on three propulsion mechanisms of micro-/nanorobots in oscillation magnetic fields: (1) wagging propulsion; (2) surface walker; and (3) scallop propulsion. Finally, a summary table is provided to compare the abilities of different micro-/nanorobots driven by oscillating magnetic fields.

Keywords: fabrication techniques; micro-/nanorobots; oscillating magnetic fields; propulsion mechanisms.

Figure 1

The categories of magnetic propulsion. Adapted with permission from Reference [84], copyright Small 2016; adapted with permission from Reference [85], copyright Advanced Functional Materials 2018; adapted with permission from Reference [83], copyright Nano Letter 2017; adapted with permission from Reference [100], copyright Advanced Materials 2013; adapted with permission from Reference [101], copyright Journal of the American Chemical Society 2010; adapted with permission from Reference [102], copyright Nanoscale 2014.

Figure 2

Membrane template-assisted electrodeposition of nanowires. (A) Preparation procedure of flexible metallic nanowires with polyelectrolyte hinges after membrane template electrodeposition. Adapted with permission from Reference [103], copyright Nature Nanotechnology 2007. (B) Schematic illustration of a fish-like nanoswimmer. Adapted with permission from Reference [84], copyright Small 2016. (C) Model of a freestyle magnetic nanoswimmer. Adapted with permission from Reference [83], copyright Nano Letter 2017. (D) Schematic representation of a magnetic multilink nanoswimmer. Adapted with permission from Reference [104], copyright Nano Letter 2015.

Figure 3

(A) Schematic illustration of the layer-by-layer assembly process. (B) SEM images of 2-link and 3-link nanoswimmers. Adapted with permission from Reference [104], copyright Nano Letter 2015.

Figure 4

(A) Schematic illustration of the electron beam evaporation process. (B) Magnified optical microscopy image and the corresponding SEM image of a microdimer, highlighting its staggered shape and the magnetic hemispheres (dark under the optical microscope, bright under the SEM). (C) Optical microscopy image of a few representative microdimers after magnetization. Adapted with permission from Reference [85], copyright Advanced Functional Materials 2018.

Figure 5

(A) Fabrication of a microrobot mold via three-dimensional (3D) printing. (B) Image of the fabrication of PDMS (polydimethylsiloxane) shells in the mold. (C) Schematic illustration of scallop swimmers. Adapted with permission from Reference [108], copyright Nature Communications 2014.

Figure 6

(A) Beating pattern of the motion of a magnetic flexible filament attached to a red blood cell. Adapted with permission from Reference [106] copyright Nature 2005. (B) Schematic showing the magnetic setup for propulsion along with the vibrating magnetic field. Adapted with permission from Reference [83], copyright Nano Letter 2015. (C) Magnetic propulsion of an artificial nanofish using a planar oscillating magnetic field. Adapted with permission from Reference [84], copyright Small 2016. (D) Image sequences of 3-link swimmers’ wagging motion. Adapted with permission from Reference [104], copyright Nano Letter 2015.

Figure 7

(A) When exposed to a magnetic field, the dimer immediately aligns its major axis with the field direction at an angle α of 47.6 ± 2.6°, determined by the dimer geometry. (B) Schematic showing the propulsion mechanism of microdimer surface walkers under an oscillating magnetic field. (C) Schematic of the experimental setup in which a magnetic dimer moves along the magnetic field direction, away from the magnet. (D) A microdimer surface walker bypasses several polystyrene microspheres. (E) A microdimer surface walker crosses continuous cracks on a glass surface. Adapted with permission from Reference [83], copyright Advanced Functional Materials 2018.

Figure 8

(A) Schematic drawing of a micro-scallop from the top view. The green shapes illustrate the opening and closing shape change of the micro-scallop when actuated by an external magnetic field. (B) Forward net displacement of the micro-scallop in a shear thickening fluid and asymmetric actuation (blue curve). (C) Forward net displacement of the micro-scallop in a shear thickening fluid with symmetric actuation (blue curve). Adapted with permission from Reference [108], copyright Nature Communications 2014.